1. Field of the Invention
This invention relates to a process for purification of substantially non-aqueous, viscous, contaminated organic liquids with reverse osmosis membranes at conditions other than "reverse osmosis conditions". More particularly, this invention relates to a process for purification of non-aqueous, contaminated higher glycols, in particular tri-ethylene glycol.
2. Description of Prior Art
This invention addresses the treatment of higher glycols, such as ethylene glycol, di-ethylene glycol, and, in particular, tri-ethylene glycol which is utilized in gas processing operations. Known methods for purifying contaminated tri-ethylene glycol include distillation, filtration and absorption with activated charcoal; however, capital and operating costs for such known processes are relatively high. In addition, filtration is only effective for suspended solids removal which is not a major pollutant of tri-ethylene glycol while activated charcoal absorption only removes hydrocarbons and, thus, is not effective in removing inorganics such as chlorides and sulfides. Removal of chlorides from tri-ethylene glycol is of critical importance.
It is also known to employ reverse osmosis membranes in purifying antifreeze, which is dilute ethylene or di-ethylene glycol. However, antifreeze which is recycled for purification is in a dilute state, typically containing 40-70% water, rendering it much easier to process than non-aqueous, contaminated glycols. Undiluted glycols are much more viscous, requiring specialized equipment assembly and processing techniques.
U.S. Pat. No. 4,902,417 to Lien teaches a spiral-wound membrane cartridge for use in ultrafiltration and reverse-osmosis separation equipment which includes a feed layer, a membrane layer and a permeate carrier layer. Such cartridges are indicated to be particularly useful with feed mixtures including suspended solids, as well as with aqueous mixtures of partially soluble solids, such as glycols, oils and proteins.
U.S. Pat. No. 3,651,617 to Hodgson teaches a regeneration system employing reverse osmosis for recovery of desiccants such as ethylene glycol, di-ethylene glycol and tri-ethylene glycol used for dehydrating natural gas from hygroscopic liquids. In accordance with the teachings of the '617 patent, a water-rich desiccant is pressurized in excess of its osmotic pressure and passed through a semi-permeable membrane of selective permeability, thereby removing water from the water-rich desiccant.
U.S. Pat. No. 4,000,065 to Ladha et al. teaches a process and apparatus for separating aqueous streams contaminated with minor amounts of organic materials using reverse osmosis. The use of reverse osmosis in conjunction with separation of lower glycols from relatively dilute solutions is also taught by U.S. Pat. No. 5,281,337 to Chou et al. which teaches a membrane and membrane support layer for separating charge systems such as aqueous mixtures of ethylene glycol in a reverse osmosis module; U.S. Pat. No. 5,167,826 to Eaton which teaches a process for recycling used engine coolant, that is glycol-based coolants, using reverse osmosis, the disclosed membrane allowing ethylene glycol and water to pass through at pressures at between 50 and 500 psi and temperatures preferably below 100.degree. F.; and U.S. Pat. No. 5,194,159 to George et al. which teaches a process for reclaiming lower glycols from operative fluids, such as antifreeze, using reverse osmosis, where the operative fluid preferably comprises less than 80% lower glycol, in which the feed stream of operative fluid to the membrane is provided at pressures between 10 and 2000 psig and temperatures between below ambient and about 120.degree. C.
Non-aqueous, that is substantially undiluted, tri-ethylene glycol, di-ethylene glycol and ethylene glycol, tri-ethylene glycol being the most viscous of the three compounds, are much more viscous than, for example, antifreeze. Accordingly, a reverse osmosis system and process designed to treat used antifreeze is not capable of processing any of the contaminated ethylene glycol compounds in a substantially undiluted state.